An appropriate quenching technique has always been an extremely important part of the heat treatment process of metals. Expensive, high value treated parts could result damaged if insufficient attention is paid to proper quenching procedure and means. The choice of the operative tempering conditions is therefore essential in view of the structural features and the technological aims which have to be reached.
Selection of a quenching agent is primarily governed by the processing specifications, the required physical properties, and the required microstructure. Due to its versatile quenching performance, oil is the most widely used quenching medium, next only to water. The worldwide requirement for quenching oil today is estimated at between 50 million and 100 million gallons per year.
Among the various quenching media, oil continues to be favored because its quenching mechanism and cooling curves are well suited to the TTT (time, temperature, and transformation) and CCT (continuous cooling transformation) diagrams of many types of steel.
Quenching of steel in liquid medium consists of three distinct stages of cooling: the vapor phase, nucleate boiling, and the convective stage. In the first stage, a vapor blanket is formed immediately upon quenching. This blanket has an insulating effect, and heat transfer in this stage is slow since it is mostly through radiation. As the temperature drops, the vapor blanket becomes unstable and collapses, initiating the nucleate boiling stage.
Heat removal is the fastest in this stage, due to the heat of vaporization, and continues until the surface temperature drops below the boiling point of the quenching medium. Further cooling takes place mostly through convection and some conduction.
During the quenching process, there are two sorts of stresses involved: thermal stresses due to rapid cooling, and transformation stresses due to the increase in volume from austenite to Martensite microstructure. Those stresses can cause excessive distortion or even cracks. However, oil has a unique desirable cooling response in minimizing those effects. Consequently, oil will continue to be used for quenching as long as it is affordable.
For the application in heat baths there are several types of quenching oils suitable for steels with low to high hardenability. Thanks to the properties of these oils, it is possible to quench also into the Martensitic temperature range—i.e. in a range between 160 and 250° C.—with minimum distortion, while still obtaining the desired properties in metal parts.
Besides hardenability, selection of an oil formulation depends on part geometry and thickness, and the degree of distortion that can be tolerated. For example, hot oil is required for smaller parts with high hardenability to achieve the desired mechanical properties with minimum distortion.
Quenching oils are available with flash points ranging from 130° C. to 290° C. The operating temperature of the oil in an open quench tank is normally at least 65° C. below its flash point. When the quench tank is operated under a protective atmosphere, oil can be used at as high as 10° C. below the flash point. The operating range of a heat bath quenching oils is normally from 10° C. to 230° C.
A lower operating temperature is in any case helpful in minimizing thermal degradation of the oil.
Originally, oil was used without any additives. It was slow in cooling and susceptible to oxidation. Research was carried out to overcome these shortcomings by adding certain chemical additives to the oil. In addition, the objective was to make oil quenching more reliable and uniform, and to control the vapor phase by starting the nucleate boiling stage sooner. Consequently, the term “fast oil” is applied to oil with such additives. Some oils also have additives that extend the nucleate boiling stage to achieve deeper hardening for some steel. Specially formulated oils also are available for vacuum heat-treating operations.
The use of vegetable oils mixtures for quenching purposes is described for instance in the patent application WO2004/099450 disclosing a vegetable quenching oil composition and additive substances which should achieve the stabilization of the chemical and technological properties of the mixtures.
However, although the benefits of using vegetable oils are various, specifically, safety, disposal, and availability, there are still some concerns regarding the metallurgical effectiveness and specific chemical and physical properties of the used mixture. In particular, a vegetable mixture achieves generally to obtain a controlled quick cooling of the treated metal but this leads to a considerably high percentage of creeks and deformations in the internal metal structure due to the difference between its superficial and internal temperature during quenching. In addition, the vegetable nature of the oil presents many drawbacks due to the various substances contained originally in the oil, which tends quickly to degrade and needs to be regenerated.